The present invention relates to a method for determining the cut quality of a laser cutting process, which is assessed based on the formation of solidification ridges along the cut face and/or burr formation at the bottom edge of the cut face, where in a simulation program a virtual laser cutting machine is provided that can be operated virtually using a set of values P0 from a parameter space P.
Laser cutting is an established separation process. Among the laser-aided manufacturing methods, it takes the leading position in industrial applications. From the user's perspective, high productivity with high quality is demanded for such manufacturing methods.
Currently, high quality CO2-Lasers (10μ-emitters) with a radiation wavelength of approx. 10 μm and with a laser power of 1-6 kW are employed industrially for laser cutting in the field of macro applications for sheet metal thicknesses ranging from 1 mm to 30 mm. In addition, new laser sources are available today, such as fiber lasers and disc lasers (1μ-emitters) with a radiation wavelength of about 1 μm, with a laser power of currently 1-8 kW, and a much better beam quality than the CO2 lasers. Such 1μ-emitters offer significant economic advantages over the established 10μ-emitters. However, 1μ-emitters have a poorer cut quality compared to 10μ-emitters, which is an obstacle to the use of 1μ-emitters.
The quality of cuts along a workpiece can be assessed based on the morphology of a ridge structure that forms on the cut face and a burr formation due to molten material at the lower edge of the cut face. Low ridge and burr formation are required in addition to flatness and squareness of the cut face.
The process chain ‘cutting-welding’ is one example in which the significance of the quality of the cut face for preparing the joining gap can be recognized. To be able to generate slim welding seams with a laser, where said seams require no post-processing by grinding or dressing, cuts of the components to be joined having plane, right-angled, smooth cut faces that are burr and oxide free are desired.
The mechanism that leads to the formation of ridges and burrs as well as oxide layers along the cut face and the cut edge of the workpiece was examined for the above reasons.                Ridges arise at the cut face and the amplitude of the ridges increases abruptly at a certain depth of the cut, i.e., a change occurs from finer to coarser ridges.        The amplitude of the coarse ridges becomes greater with an increasing thickness of the material to be cut.        The coarser ridges are interrupted repeatedly or exhibit irregular spacing (number of ridges in the cutting direction changes with the depth of the cut).        
This axial structure or an interruption of the ridges produces an irregular structure of the cut face and is undesirable. Today, the achievable ridge amplitude is smaller for the 10μ-emitter than for the 1μ-emitter.                Ridges with the greatest amplitudes caused by the solidification of molten metal on the cut face occur especially in the lower part of the cut face or with large material thicknesses.        In particular with high feed rates the melt does not fully come off the bottom edge. The attached and then solidifying melt forms the undesired burr. The mechanisms for the burr formation are understood only to a certain degree, which means that the potential productivity values of cutting equipment is significantly under-utilized today.        The formation of cracks and pores in the weld seam can be caused by oxidized joining edges, as they occur during flame cutting. For this reason, fusion cutting is performed with an inert cutting gas to obtain oxide free cut faces.        
Document EP-B1 0929 376 describes a method for laser beam treatment, which is said to be particularly suitable for cutting large material thicknesses of 15 mm or greater. According to this method several foci are created, which are positioned in the axial direction along the thickness of the material to produce a large depth effect of the laser radiation. However, it appears that despite the measures recommended in this document, the formation of ridges and burrs occurs with an unchanged severity. Also, the portions of the laser radiation with a deeper focus lead to an unwanted expansion (rounding) of the kerf on the upper edge of the material.
The current state-of-the-art is not sufficient to establish a quality cut using the 1μ-emitter for a sheet thickness of more than 2 mm and to expand the quality cut to more than 15 mm sheet thickness using the 10μ-emitter. The reasons for these technical limitations are that                an extension of the limits for the cut quality cannot be achieved according to the present experimental experience using 1μ-emitters and 10μ-emitters and with today's available laser and process parameter settings        the existing understanding about the formation of ridges and burrs is insufficient to recognize, for example, the necessary, fundamental new effects of a beam-shaping and then specify beam-shaping measures.        
For these reasons, experts today only propose measures to improve the cutting process using 1μ-emitters, which are known from the experiences of cutting using 10μ-emitters. Thus far, these measures have been unsuccessful and the 1μ-emitter still cannot achieve the cut quality of the 10μ-emitter. In addition, no physical cause is known that could explain the different cut quality.
The state of the technology and science verifies that at least two types of ridges exist, namely melting ridges and solidification ridges with their morphology giving an indication of the underlying formation mechanism.
Among others, the document, Schulz W. entitled “Simulation of Laser Cutting” in The Theory of Laser Materials Processing, edited by J. Dowden, Springer Series in Materials Science, 2009, Vol. 119, P. 21-69. ISBN 13 978-1-4020-9339-5, describes the simulation of laser cutting. Differential equations are listed for this, among other things. The document also deals with the Weber number. Among others also, the document, Schulz W. entitled “Dynamics or ripple formation and melt flow in laser beam cutting”, in J. Phys. D: Appl. Phys., 1999, Vol. 32, P. 1219-1228, examines the dynamic behavior of ridge formation and the melt flow during laser cutting of metals. According to the information given in these two documents, it is possible to calculate the ridge formation during laser cutting, i.e., always by specifying interferences that act on the system from the outside. Only melt ridges are observed according to these documents.
Melting ridges form on the upper side of the cut face solely by movement of the melt front in the absence of solidified melt and have a small amplitude compared to solidification ridges and are technically of minor significance.
Solidification ridges form in greater cutting depth, typically 1 to 2 mm below the top edge of the sheet being cut, by wave-like formation of the melt front and by wave-like solidifying melt and have a great amplitude compared to the melting ridges and are technically very significant.
According to the state-of-the-art and science, the correlation between the ridge and burr formation on the one hand and the laser, machine and material parameters on the other hand is not clarified. For this reason, making a quality cut using the new laser sources (e.g., fiber lasers) is still not mastered, which prevents the wide application of the new radiation sources and is the subject of worldwide research.